3Mask technology uses a Lift-off process to simultaneously form an indium tin oxide (ITO) layer (pixel electrode) and a passivation (PV) layer using a single mask, thereby a total number of masks can be reduced to three masks.
A traditional 3Mask process is primarily for a twisted nematic (TN) mode, wherein the ITO does not form a slit pattern. Even if the ITO forms a slit pattern, because the ITO can only be deposited into a dug hole, the entire ITO layer is inside a SiNx (silicon nitride) groove, so that a horizontal electric field of the ITO is weakened, therefore influencing the liquid crystal display, and causing unevenness display brightness. However, with development of the technology, in an improved 3Mask technology, PV/ITO layers are formed by a halftone mask (HTM mask) or a gray scale mask (GTM mask), so that an ITO of a pixel area can not only form a slit, but can also cover a PV layer, to form a structure the same as a 4Mask structure.
In this type of large area HTM mask, because it adopts a semipermeable membrane with a constant light transmittance, during a single-layer film experiment, the mask has a narrower slit causing lights to be partially diffracted, so that an amount of sensitivity of a photoresist in a corresponding area is less than an amount of sensitivity of the photoresist in other areas, and the difference of the amounts of sensitivity causes a difference of film thicknesses between the two locations of the photoresist of about 0.5 um. During a full process experiment, because a light-reflection from a drain electrode metal under a via of an array substrate, an amount of exposure of a mask in a corresponding area above the drain electrode metal is enhanced, so that the thinner photoresist in the corresponding area is thinned again, or even disappears, thereby in a following dry etching process, the ITO will be etched and weakened, causing disruption in the ITO from lower to higher surfaces.
As mentioned above, in a traditional HTM mask, because each area of the semipermeable membranes adopts the same light transmittance, when manufacturing an array substrate, film thicknesses of a photoresist in different areas on a surface of one ITO layer are different, this even causes the thinner photoresist to disappear, so that in a following dry etching process, the ITO will be etched and weakened, causing disruption in the ITO from lower to higher surfaces.